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Reverse Linked List

Problem​

Given the head of a singly linked list, reverse the list, and return the reversed list.

 

Example 1:

Input: head = [1,2,3,4,5]
Output: [5,4,3,2,1]

Example 2:

Input: head = [1,2]
Output: [2,1]

Example 3:

Input: head = []
Output: []

 

Constraints:

  • The number of nodes in the list is the range [0, 5000].
  • -5000 <= Node.val <= 5000

Solution​

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var reverseList = function(head) {
    let prev = null;
    let cur = head;
    while (cur) {
        [cur.next, cur, prev] = [prev, cur.next, cur];
    }
    return prev;
};

Keep track of the previous node prev and the current node cur while reversing the list iteratively.

Follow-up​

A linked list can be reversed either iteratively or recursively. Could you implement both?

Solution​

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var reverseList = function(head, prev = null) {
    if (!head) {
        return prev;
    }
    const next = head.next;
    head.next = prev;
    return reverseList(next, head);
};

Reverse each node recursively and return the new head (ie. old tail) at the end.